Directing an optical instrument such as binoculars toward an intense light source such as the sun can be very hazardous to the eyes. In astronomical instruments, this problem is sometimes solved by placing in the focal plane inside the instrument, an opaque disk positioned exactly at the focused image of the sun. The disk has the same dimension as the image of the sun and thus blocks out the solar rays. In energy detection devices such as the ones described in U.S. Pat. No. 3,020,406 by T. R. Whitney and U.S. Pat. No. 3,714,430 by R. C. Finvoid, a photochromic layer is placed at the focal plane of the device in order to protect sensitive photodetectors. The high energy rays cause the photochromic material to increase in opacity at those points where the rays are most intense. Thus, high intensity rays are attenuated by the same dark spots they generate, and dim rays are left unaffected. Many photochromic chemicals and systems are available. A technique for generating photochromic chemical is described in U.S. Pat. No. 5,062,693.
These prior art devices make use of photochromic material sensitive to infrared radiation, such as germanium and gallium arsenide. Furthermore, these devices are specifically designed to protect inanimate energy detectors and transducers.
There is a need to reduce glare in conventional instruments such as rearview mirrors, binoculars, welding visors and periscopes, operating with classical optics and visible light, and specifically, to protect human vision from high intensity light sources such as the sun, car headlights and welding arcs. In addition, the size of the instruments should be minimized by folding the optical path. As is well known in optics, this can be achieved by the use of prisms and mirrors.